1. Field of the Invention
The present invention relates to a technique of a light emitting device, more specifically, the invention relates to a light emitting device and a driving method therefor.
2. Description of the Related Art
Recently, display devices for performing image display are being developed. Liquid crystal display devices that perform image display by using a liquid crystal element are widely used as display panels for mobile phones and display devices for personal computers because of advantages of high image quality, thinness, lightweight, and the like.
In addition, light emitting devices using self-light emitting elements as light emitting elements are recently being developed. The light emitting device has characteristics of, in addition to advantages of existing liquid crystal display devices, for example, a high response speed suitable for dynamic image display, a low voltage, and low power consumption Therefore, the light emitting device is expected to have a wide range of applications including new generation's mobile phone and personal digital assistance (PDA), thereby attracting a great deal of attention as the next generation display device.
The light emitting element is also called organic light emitting diode (OLED), and has a structure of an anode, a cathode, and an organic compound layer between the anode and the cathode The current flowing to the light emitting element is in directly proportional to the luminance of the light emitting element, the light emitting element emits light corresponding to the amount of the current flowing to the organic compound layer.
A time-gray-scale scheme is adopted for the light emitting device (For example, refer to Patent Document 1). Further, a method of applying reverse biases to the light emitting element also can be adopted (For example, refer to Patent Document 2).
[Patent Document 1]
JP 2001-5426
[Patent Document 2]
JP 2001-142413
However, the light-emitting element is not resistive to the moisture or oxygen in air, thus involving a problem of low reliability, heat-resisting stability, durability and the like due to the deterioration in an organic compound layer. For this reason, there is a proposal that a light emitting element is to be applied by a drive voltage having a polarity reverse to that upon light emission (reverse bias voltage) during each of constant time periods. This is because the light emitting element is improved against the deterioration in current-voltage characteristic by applying such a reverse polarity of drive voltage to the light emitting element.
In order to apply a reverse polarity of drive voltage to the light emitting element, there is a need to change the potential at between the first and second electrodes of the light emitting element. The simplest way for changing the potential at between the first and second electrodes is to change a counter potential of the light emitting element. However, the counter potential of a light emitting element, in many cases, is connected to a line common to all the pixels, making it impossible to change the counter potential pixel by pixel or line by line. Namely, to change the counter potential of a light emitting element, there is no way but to carry out at one time on all the pixels. Thus, there is a difficulty in changing the timing of the counter potential. Accordingly, in case a reverse bias is to be applied by changing the counter potential of the light emitting element, there encounters an influence upon gray scale representation.
Meanwhile, there are various schemes of drive methods to display multi-gray-scale image on a light-emitting device using light-emitting elements, one of which is a voltage-input scheme. The voltage-input scheme means a scheme that a video signal, for input to the pixel, is inputted to a gate electrode of a drive element thereby controlling the brightness on the light-emitting element through the use of that drive element.
However, in the case of the voltage input scheme, the semiconductor element for driving the light-emitting elements is formed of polycrystal semiconductor (polysilicon) having a high on-current. However, the polysilicon transistor formed of polysilicon involves a problem that its electrical characteristic readily varies due to the defects in grain boundaries. In case there is variation in characteristics, such as threshold or on-current, pixel by pixel on the transistors configuring the pixels, even when inputting the same video signal, the drain current though the transistor is different correspondingly thus resulting in brightness variation between the light-emitting elements. Furthermore, there occurs unevenness in the emission-light brightness on the pixels of the screen, resulting in blurs.
Accordingly, it is a subject of the present invention to provide a light-emitting device which is to be applied by a current input scheme capable of controlling the magnitude of a current flowing through the light-emitting element not dependent upon characteristics of the transistors configuring the pixels.
Also, it is a subject to provide a light-emitting device that a reverse bias is applied to the light-emitting elements freely from the influence upon gray scale representation thereby improving against the deterioration in current-voltage characteristics.